Consider the following equation:
2SO₂(g)+O₂(g)⇌ 2SO₃(g),Δ H=-190 kJ
The number of factors which will increase the yield of SO₃ at equilibrium from the following is _______.
A. Increasing temperature
B. Increasing pressure
C. Adding more SO₂
D. Adding more O₂
E. Addition of catalyst

Solution

Chemistry ma’am · 2 steps

Ask of each change which way it pushes the equilibrium; a catalyst pushes neither way.

1. Read the reaction. The forward reaction is exothermic (Δ H = -190 kJ). It also turns 3 moles of gas into 2 moles of gas.

   Change	How the equilibrium responds	More SO₃?
   A. Raise temperature	Shifts backward, the endothermic direction, to absorb the heat	No
   B. Raise pressure	Shifts forward, to the side with fewer gas moles (2 rather than 3)	Yes
   C. Add SO₂	Shifts forward to use up the extra reactant	Yes
   D. Add O₂	Shifts forward to use up the extra reactant	Yes
   E. Add catalyst	Speeds up the forward and backward reactions equally, so equilibrium only arrives sooner	No

2. So. B, C and D increase the yield of SO₃. The answer is 3.

The common slip: 4. Counting the catalyst as a factor gives 4. It speeds up both directions equally, so the yield does not change.

The idea to keep. Le Chatelier tells you where equilibrium moves, so only changes that move it alter the yield. A catalyst changes how fast equilibrium is reached, never where it sits.

Answer from the NTA final answer key. Solution by PrepWiser.